It is well known to test work pieces of various kinds by means of ultrasonic waves to locate unsound spots, or flaws, in the otherwise homogeneous structure. Depending upon the type of material and its form, methods were developed in order to test by means of an echo signal or an emitted ultrasonic impulse which is received by a transmitting-receiving transducer or, after emission from a pure transmitter, by a receiver. It is possible to use an arrangement wherein the transmitter is on one side of the work piece and the receiver on the other side, in which case an expert in this field speaks of a transmission technique wherein the intensity of the received ultrasonic energy is a measure of the perfection. On the other hand, when one uses an arrangement wherein the transmitter and receiver are both disposed on the same side of the work piece, then one speaks of an echo process wherein the temporal distribution of returning sound waves reflected from interfaces in the material is exploited.
In German Offenlegungsschrift No. 24 50 402, for example, a testing system employing the pulse-echo process and operating with several frequencies is disclosed. That system uses a testing head and several pulse transmitters by which signals of several frequencies can be produced which penetrate into different zones of the material in order to be able to deduce the location of faults in the cross section under test.
The thickness and the surface configuration of a wall which is inaccessible on one side can be measured by a process, for example, such as that shown in German No. OS 27 40 106 wherein the orientation of the rear wall can be determined by optimization of the receiving angle.
Another method for localizing faults is shown in German OS No. 20 06 110, according to which deductions can be made determining the location of a fault by setting an adjustable delay for one or two quasi stereo signals.
Finally, German Auslegeschrift No. 15 73 409 shows a carriage which carries test heads for the ultrasonic testing of railroad rails, the carriage being guided along the rails with the heads in order to carry out a continuous testing procedure.
All of these processes have the disadvantage that, although one can determine the location of a flaw, it is not possible to determine the form or shape of the flaw, or projecting portions thereof unless the work piece is scanned from several sides in accordance with known methods employing ultrasonics, in order to reconstruct the shape of, for example, a hollow space from a large number of detailed data.